While running a long print, I noticed that my bed temperature was changing erratically with a downward trend. After about 10 minutes it just stopped heating and slowly cooled down to room temperature.
I had a spare heat bed kit (heater, glass, PEI combo), so I installed that. That didn’t resolve the problem, still no heat.
Everything else is working fine, I get heat on the extruder, and everything moves as it should. Since I did a replacement of the heater, any poor connections there would have been discovered.